1. Field of the Invention
The invention relates to a method for the preparation of easily dispersible, high color strength alkali blue pigments and the pigments produced according to this method.
2. Description of the Prior Art
"Alkali blue" has been known for a considerable length of time. In form of the internal salt it is used to a great extent as pigment for printing inks.
"Alkali blue" corresponds with the following general formula: ##STR1## In this formula, R is hydrogen, chlorine, bromine, C.sub.1 - to C.sub.4 -alkyl, C.sub.1 to C.sub.4 -alkoxy, nitro, amino, C.sub.1 - to C.sub.4 -alkylamino, R.sup.1 is hydrogen or C.sub.1 - to C.sub.4 -alkyl, R.sup.2 hydrogen, C.sub.1 - to C.sub.4 -alkyl, chlorine, bromine or SO.sub.3 H.sub.7 and R.sup.3 is hydrogen or phenyl which is possibly substituted by R. R.sup.2 is preferably a hydrogen or C.sub.1 - to C.sub.4 -alkyl.
Since the pigments of formula I are strongly polar and hydrophilic, drying of the presscakes results in hard, no longer dispersible agglomerates and aggregates. To be added to this is the fact that strong hydrogen bonds exist at the surface due to the extremely fine pigment particles. This makes it difficult to prepare a dyestuff suitable for printing by milling the dry pigment in bonding agents and/or bonding agent solutions suitable for this purpose on the triple roller grinding mill. (E. K. Fischer in American Inkmaker, volume 23 (1945), No. 12; G. R. Buckwalter in T. C. Patton, Pigment Handbook, vol. 1, page 620, John Wiley & Sons, New York 1973.)
For this reason, other methods were developed for processing "alkali blue pigments" such as the flush method. In these processes, the moist pigment, for instance, in a form of a presscake is transferred into the desired mixture of bonding agent/solvent by mixing or kneading while being wetted by the organic phase. The resultant "flush paste" can be used directly for the preparation of printing inks. As a rule, the paste prepared according to the flush method contains 35 to 40 percent by weight of pigment. A drawback of the flush method is that large and thus expensive kneaders are required for the large-scale production of pigment paste and that the paste is produced on a discontinuous basis. This results in high manufacturing costs. Another drawback of flush paste is the high (60 to 65 percent by weight) amount of bonding agent and solvent since these must be compatible with other (additional) binders and/or solvents used in the preparation of printing inks. Additional difficulties are incurred in adjusting the color strength, the viscosity, and tackiness in the completed printing inks which are required for use.
For this reason, an attempt was made to produce alkali blue preparations with higher concentrations which can be used on a broader basis. Thus, preparations are known which contain 10 to 70 percent by weight of one or more natural and/or synthetic resins which result in easily dispersible pigment powders after drying which supply printing inks with high color strength and excellent crystal hardness.
U.S. Pat. No. 4,032,357 describes the preparation of powdered alkali blue preparations by combined precipitation of the pigment and an anionic organic dispersing agent which contains at least 8 carbon atoms and which is not soluble in water in its acid form and the subsequent addition of a hydrophobic oil to the precipitate or the presscake of the precipitate.
Preparations are known from U.S. Pat. No. 3,925,094 which are obtained by precipitating a solution of alkali blue and a resin which can be precipitated with acid in an aromatic amine which is liquid to temperatures to below 50.degree. C. by discharging in aqueous acid. The preparations are easily dispersible and display high color strength.
U.S. Pat. No. 3,635,745 describes the preparation of powdered alkali blue preparations by the combined precipitation with acid of an alkaline solution containing alkali blue and an acid resin.
It is known from U.S. Pat. No. 4,189,328 to produce preparations of alkali blue by precipitating with acid (pH &lt;5) an alkaline solution of alkali blue and a phenol which is insoluble in water at pH &lt;5 and has a melting point of 35.degree. C.
It is the purpose of this invention to prepare a powdered alkali blue which is easily dispersible in the binders or binder solutions commonly used for the preparation of offset printing inks and which has a good crystal hardness.